It has long been known that implantable medical electrical leads may be positioned transvenously within one or more chambers of the heart to provide electrical stimulation to, and to monitor signals occurring within, the cardiac tissue. In order to achieve reliable sensing of the cardiac electrogram and/or to apply stimulation that effectively paces or cardioverts the heart chamber, it is necessary to accurately position the electrode surface against the endocardium or within the myocardium at the desired site and affix it during an acute post-operative phase until fibrous tissue growth occurs. After implantation, the leads may be coupled to an implantable medical device (IMD) such as a pacemaker or cardioverter/defibrillator so that the desired stimulation may be provided to the cardiac tissue.
More recently, endocardial pacing and cardioversion/defibrillation leads have been developed that are adapted to be advanced into the coronary sinus and branch coronary veins. During this type of implant procedure, a distal end of a lead is advanced through the superior vena cava and the right atrium, and through the ostium of the coronary sinus. The lead may further be advanced within the coronary sinus into one of the branch veins.
Placement of leads within the coronary sinus and branch veins is important because these leads can be located adjacent to the left ventricle or the left atrium of the heart. Electrical stimulation can then be provided to the left chambers of the heart without actually placing one or more leads into these chambers. Because the left side of the heart accounts for the majority of the heart's hemodynamic output, various pathologies may be better treated through such left-heart stimulation. For example, in patients experiencing conditions associated with heart failure, electrical stimulation of both the right and left sides of the heart can be used to re-synchronize the depolarization of the left and right ventricles in a manner that increases the cardiac output.
In addition to providing important benefits to heart failure patients, the location of electrodes within the coronary sinus and branch veins can also reduce defibrillation thresholds. That is, when a shocking electrode is positioned within a left-sided cardiac vein and used in conjunction with other shocking electrodes placed in more traditional locations, a lower shock energy may be required during cardioversion and/or defibrillation therapy. This can reduce the discomfort associated with these therapies.
Several challenges are posed by providing both pacing and defibrillation electrodes within the coronary vasculature. Because of the small vessel size, positioning multiple leads within the vasculature is difficult. Additionally, the size of coil electrodes of the type needed for high-voltage therapies may be limited based on the size of the vessels, thereby limiting the area of the tissue through which current flows during the therapy. This may limit the efficacy of high-voltage therapies. Finally, locating the leads at a precise location needed to provide adequate tissue stimulation may be difficult given the problems associated with navigating the torturous vessels such as the coronary sinus and branch veins.
What is needed, therefore, is an improved system that may be used to provide both pacing and high-voltage therapy to the left chambers of the heart and that may be reliably fixed within a branch vein of the coronary sinus.